Haemophilus influenzae type b is the most common cause of bacterial meningitis and other serious invasive bacterial infections in children. There has been controversy about the protective efficacy and cost-effectiveness of the plain polysaccharide vaccine against H. influenzae type b that was licensed in 1985, in part because of the paucity of data on its efficacy in U.S. populations. In December 1987 a protein-polysaccharide conjugate vaccine against H. influenzae type b (PRP-D) was licensed for use in children at 18 months of age. The conjugate vaccine has essentially replaced the plain polysaccharide vaccine, although there are no data on its clinical efficacy in children 18 months of age and older. It is expected that PRP-D and/or other conjugate vaccines soon will be licensed for use in infants beginning at 2 months of age. The purpose of the proposed case-control study is to assess the protective efficacy of conjugate vaccines against invasive H. influenzae type b infections among children in Connecticut and greater Pittsburgh, Pennsylvania. All children with H. influenzae type b isolated from normally sterile sites will be detected through prospective, active surveillance of the microbiology laboratories and hospitals in both study areas. Cases will be children without underlying diseases who are 18 months of age or older (2 months of age or older once conjugate vaccines are licensed for infants), and who develop invasive infections with H. influenzae type b. For each case, two controls, matched to the case by age and town o? birth, certificate records. Parents of both Antecedent will be selected randomly from birth certificate demographic information and controls will be interviewed to obtain socio-immunization with Haemophilus vaccines will be ascertained from the written records of all sources of medical care. Estimates of the vaccine's protective efficacy will be made from the matched odds ratios with their associated 95% confidence intervals. These estimates will be adjusted for the possible effects of confounders with the use of conditional logistic regression. The study will have more than 90% statistical power to detect a protective efficacy of the vaccine of 50% or more. The results of this study will have important implications for public health and immunization policies.